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 Table of Contents  
ORIGINAL ARTICLE
Year : 2016  |  Volume : 1  |  Issue : 1  |  Page : 33-38

Environmental factors other than iodine deficiency in the pathogenesis of endemic goiter in the basin of river Ganga and Bay of Bengal, India


1 Department of Physiology, Endocrinology and Reproductive Physiology Laboratory, University of Calcutta, Kolkata, West Bengal, India
2 Department of Medicine, Section of Endocrinology, Diabetes and Nutrition, Boston University School of Medicine, Boston, Massachusetts, USA

Date of Submission15-Apr-2016
Date of Acceptance10-May-2016
Date of Web Publication2-Jun-2016

Correspondence Address:
Amar K Chandra
Department of Physiology, Endocrinology and Reproductive Physiology Laboratory, University of Calcutta, Kolkata, West Bengal
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2456-1975.183283

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  Abstract 

Background: In iodine-replete basin of the river Ganga and the Bay of Bengal, we studied iodine nutritional status of school children by goiter prevalence and their urinary iodine (UI), iodine content in edible salt, and the bioavailability of iodine through water and its contribution to iodine nutrition. We also studied consumption pattern of common goitrogenic plants by measuring urinary thiocyanate (USCN), hardness of water (calcium and magnesium salt content) and assessed the effect of concomitant exposure of those environmental factors in goitrogenesis.
Methods: 4603 children aged 6-12 years were examined for goiter by palpation, 520 urine samples were analyzed for UI and USCN; iodine content was estimated in 455 household salt and 130 water samples tested both for iodine and hardness.
Results: The total goiter rate was 35.9%, median UI was 231 ΅g/l, mean USCN was 0.857 ± 0.48 mg/dl, iodine content in water was 44.7 ± 4.1 ΅g/l, 66.4% of salt samples contained iodine (15 ppm), and water was found to be hard. UI was correlated with both the drinking water iodine content and USCN and the degree of hardness in drinking water was associated with goiter prevalence.
Conclusions: The studied population has endemic goitre despite iodine sufficiency. The concomitant exposure of a number of environmental factors, i.e., thiocyanate of cyanogenic plant food, hardness of drinking water, and excess iodine from environmental sources other than iodide salt are likely responsible for the causation and persistence of endemic goiter in the region.

Keywords: Basin of Ganga, endemic goiter, environmental goitrogens, excess iodine, thiocyanate, water hardness


How to cite this article:
Chandra AK, Debnath A, Tripathy S, Goswami H, Mondal C, Chakraborty A, Pearce EN. Environmental factors other than iodine deficiency in the pathogenesis of endemic goiter in the basin of river Ganga and Bay of Bengal, India. BLDE Univ J Health Sci 2016;1:33-8

How to cite this URL:
Chandra AK, Debnath A, Tripathy S, Goswami H, Mondal C, Chakraborty A, Pearce EN. Environmental factors other than iodine deficiency in the pathogenesis of endemic goiter in the basin of river Ganga and Bay of Bengal, India. BLDE Univ J Health Sci [serial online] 2016 [cited 2019 Jul 17];1:33-8. Available from: http://www.bldeujournalhs.in/text.asp?2016/1/1/33/183283

Implementation of the universal salt iodization program in a region without the proper knowledge to its environmental iodine status is a major concern since excess iodine ingestion in several regions during post salt iodization phase is associated with chronic autoimmune thyroiditis, hypothyroidism, and other thyroid disorders. [1],[2] A retrospective study in school children from the Sundarban delta in West Bengal reported a high prevalence of goiter (38.2% and 33.1%, respectively, in South 24 and North 24 Parganas Districts of Sunderban delta) though they had no nutritional iodine deficiency as evidenced by their urinary iodine (UI) excretion. [3],[4] Based on our preliminary survey of iodine deficiency disorders (IDDs) in the basin of the river Ganga and the Bay of Bengal, 13 Community Development (CD) blocks were selected on opposite sides of the Sundarban delta. Besides examining iodine nutritional status by goiter prevalence and UI excretion pattern, distribution of iodine through household salt, consumption of common goitrogenic plant foods (thiocyanate precursors), and hardness of drinking water (presence of total calcium and magnesium salts in water), since it has been hypothesized that all these may interfere iodine metabolism. Further, the correlation between drinking water iodine content and UI content to understand the contribution of environmental iodine in iodine nutrition, association between urinary thiocyanate and UI levels to understand the impact of thiocyanate in iodine excretion, and to study the relationship between hardness of drinking water and total goiter rate to understand the involvement of water hardness in goitrogenesis. Finally to examine the combined effects of those pro-goitrogenic agents found in food and water of the region on thyroid disruption.


  Materials and Methods Top


Selection of study areas

Thirteen CD blocks located in the basin of the river Ganga and the Bay of Bengal in the Howrah and Purba Midnapore districts of West Bengal on the opposite sides of the Sunderban delta were selected. The total population of this region is 31, 50, 357 (2001 census). Most of the population is engaged in agricultural activities, fisheries, and to a certain extent, working in factories such as the Haldia oil refinery. Local diets are mainly nonvegetarian and consist primarily of cereals (rice), pulses, fish, and vegetables of the Brassica family. The study was conducted between November 2010 and August 2013.

Selection of population

To obtain proper representation, one locality was randomly selected from each of thirteen CD blocks. [5] In each locality, one primary school and the nearest adjoining secondary school were selected at random [6] so that students aged 6-12 years of both sexes would be available as recommended by the WHO/UNICEF/ICCIDD. [7] Ethical clearance for the study was obtained from the Institutional Ethical Committee.

Clinical goiter survey

A total of 4603 students were examined for goiter by palpation. The clinical examination for goiter was conducted by two trained research staffs experienced in IDD survey methodology. Goiter grading was done according to the criteria recommended by WHO/UNICEF/ICCIDD [7] (Grade 0: No goiter; Grade 1: Thyroid palpable but not visible; and Grade 2: Thyroid visible with the neck in normal position). The age of the students was recorded from the school register and was rounded to the nearest whole number.

Iodine and thiocyanate in urine

Five hundred and twenty spot casual urine samples were collected from the examined children (40 from each locality), maintaining proportionate representation from the entire population of the studied school(s). [8] A drop of toluene was added to each urine sample to inhibit bacterial growth and to minimize odor. Iodine concentrations in urine were determined by the arsenite method following dry ashing in the presence of potassium carbonate. [9] Thiocyanate content in the urine was measured from the same urine samples analysis by the method of Aldridge [10] as modified by Michajlovskij and Langer. [11]

Iodine in salt and water

Local sources of dietary iodine are water, food, and iodized salt. To monitor the iodine content of salt available in the area, 35 marked air tight plastic containers were distributed at random to students of the studied schools in each locality, and they were asked to bring edible salt samples from their households the next day. The salt samples were kept at room temperature in the laboratory and iodine content was measured within a week following the iodometric titration method. [12] One hundred and thirty drinking water samples were collected at random (10 samples from each area) [5] and kept at 4°C until iodine concentrations were measured using the method of Karmarkar et al. [9]

Hardness of drinking water

Hardness of drinking water (total calcium and magnesium salt content) was measured from the 130 drinking water samples. On the day of analysis, all the collected ten samples in an area were mixed properly and then hardness was measured following the EDTA titration method. [13]

Statistical analysis

Descriptive data include means, standard deviations, medians, and ranges, as appropriate. Pearson correlations were used to examine univariate associations.


  Results Top


Goiter prevalence

A total of 4603 children were examined in the age group 6-12 years. Out of 4603 school children (50.9% were boys and 49.03% were girls), goiter was detected in 1656 (35.9%) of them. Grade1 goiter was present 31.5% and Grade 2 was present in 4.4% of the children [Table 1]. The studied population of all the 13 localities in coastal districts was homogenous with respect to geographical characteristics, economic status, and dietary practices.
Table 1: Goitre prevalence of different study localities in the basin of river Ganga and Bay of Bengal, India

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Urinary iodine and thiocyanate

Median UI concentrations of the studied localities ranged from 132.5 to 350 μg/L in 520 urine samples, indicating that the population had no nutritional iodine deficiency as median UI levels were above 100 μg/L in all study localities. The mean urinary thiocyanate levels ranged from 0.656 ± 0.359 to 1.130 ± 0.620 mg/dl in the studied urine samples. The median and mean iodine and thiocyanate level of the overall studied population were 231.3 μg/l and 0.857 ± 0.486 mg/dl, respectively [Table 2].
Table 2: Urinary iodine and thiocyanate excretion of studied population, iodine content in salt, iodine content and total hardness in drinking water of different study localities in the Basin of river Ganga and Bay of Bengal, India

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Iodine content in edible salt and water

The proportion of salt samples containing ≥15 ppm iodine ranged from 40 to 94.5%; overall, 66.4% households were consuming salt with adequate iodine. In other words, in most of the study localities iodine supplementation through household salt was not satisfactory [Table 2]. However, no salt sample was found without iodine.

A total of 130 drinking water samples were collected from 13 study localities taking 10 samples from each locality and were analyzed for iodine content which were in the range of 6.9±1.6 - 92.5±5.1 ΅g/l with a mean iodine level of 44.7 ± 4.1 μg/l [Table 2].

Hardness of water

The hardness of drinking water samples (total calcium and magnesium salts present) collected from the different study localities ranged from 210 ± 15.6 ppm to 618 ± 23.3 ppm [Table 2].

Correlation analyses

Correlation between iodine content in drinking water and UI content of the population, between urinary thiocyanate excretion levels and UI level, and between the hardness of drinking water and the total goiter rate of the population are shown in [Figure 1],[Figure 2] and [Figure 3], respectively. A positive correlation (r = 0.472; P < 0.05) was found between drinking water iodine content and UI concentrations of the population [Figure 1]. Further, a significant positive correlation (r = 0.158; P < 0.001) was found between urine thiocyanate and iodine concentrations [Figure 2]. Finally, a highly significant positive correlation (r = 0.544; P < 0.001) was found between the total hardness of drinking water and goiter prevalence [Figure 3].
Figure 1: Relationship between drinking water iodine content and urinary iodine concentrations of the population in the basin of the river Ganga and Bay of Bengal

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Figure 2: Relationship between urinary thiocyanate excretion and urinary iodine levels of the population in the basin of the river Ganga and Bay of Bengal

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Figure 3: Relationship between drinking water hardness and total goiter rate of the population in the basin of the river Ganga and Bay of Bengal

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  Discussion Top


The purpose of this study is to explore the combined effects of a number of environmental goitrogenic agents found in food and water on thyroid disruption developing goiter in the population of the basin of the river Ganga and Bay of Bengal in the coastal region of West Bengal.

In general, the size of the thyroid gland is inversely associated with iodine intake, with a lag interval that varies from a few months to several years, depending on many factors. These include the severity and duration of iodine deficiency, the type and effectiveness of iodine supplementation, and possible additional goitrogenic factors (WHO/UNICEF/ICCIDD). [7] The overall total goiter rate of 35.9% among school-children in our study indicated that endemic goiter/IDD is severe and a major public health problem in the basin of river Ganga and Bay of Bengal. In 11 out of 13 studied localities, the prevalence of goiter (>30%) indicated that IDD was severe and other two localities it was moderate, as goiter prevalence was >20%.

About 90% of ingested iodine eventually appears in urine and thus UI excretion is a good marker of very recent dietary iodine intake. In individuals, UI excretion typically varies from day to day and even within a given day. [14] In general, 30 urine iodine concentrations, from a defined sampling group are sufficient to understand the status of iodine nutrition (WHO/UNICEF/ICCIDD). [7] The RDA for iodine is 150 mcg in adults. [15] In school-aged children, median urinary concentrations between 100 and 199 μg/l define a population having adequate iodine nutrition but concentrations in between 200 and 299 μg/l are consistent with more than adequate, and median UI concentrations ≥300 μg/l are considered to be excessive and are associated with risk for adverse health consequences (iodine-induced hyperthyroidism, autoimmune thyroid diseases, etc.). [7] Of the 13 studied localities, 3 localities had adequate iodine nutrition, six had more than adequate iodine nutrition, and in the remaining four localities iodine nutrition was excessive. The excessive environmental iodine present in the water of this coastal region contributes iodine more than adequate level and thus responsible for thyroid enlargement [Figure 1]. Excess iodine may exist naturally such as that reported in costal Hokkaido in Japan where goiter was found to be endemic among the children. [16] Further, an association between excess iodine intake, increased serum thyroid stimulating hormone (TSH) and thyroid volume was detected in China where drinking water is rich in iodine. [17] Excess iodine consumption can also result from additional supplies of iodine for intake in the different form of fortified food. [18] Distribution of iodized salt poorly monitored for the iodine concentration resulted in excess iodine intake in Iceland. [19]

It has been recommended that at least 90% of the households should have access to iodized salt at the recommended level of 15 ppm. [7] This study showed that in overall only 66% of total households were consuming salt with adequate iodine [Table 2]. Therefore, the supply of dietary iodine through salt was not satisfactory; however, the iodine nutritional status of most of the studied population in the study localities was above requirement and even excessive, indicating that the population must have other significant sources for iodine intake. The people of this coastal area are mainly rural, and they consume foods that are largely cultivated locally. Iodine content in the agricultural products of a region is known to be dependent on iodine content in the soil. Iodine content in the drinking water indicates that this coastal region is environmentally iodine sufficient or the soil rich in iodine. [20] Drinking water iodine content of the study localities found positively correlated with UI concentrations of the population. Therefore, iodine present in drinking water not only compensates the inadequacy of iodine in locally-available iodized salt but also supplies more than adequate iodine to the population.

The prevalence of goiter among the studied population was high despite the supplies of adequate dietary iodine, suggesting the presence of dietary goitrogens and/or naturally occurring antithyroidal/goitrogenic agents in food and drinking water. To look into this aspect, we measured the thiocyanate excretion pattern of the population and hardness of drinking water.

Thiocyanate, one of the best known and widely studied goitrogen responsible for causation or aggravation of endemic goiter especially in a relatively or severely iodine-deficient region. [21] However, a successful salt iodine fortification program failed to prevent goiter development in school children from Manipur in Northeast India due to the high level of consumption of bamboo-shoots, food containing thiocyanate and thiocyanate like compounds. [22] Thiocyanate has also been implicated as goitrogenic factor in Tripura, [23] another northeastern state of India, sub-Himalayan tarai region in Uttar Pradesh [6] and even in the iodine replete Sundarban delta of Gangetic West Bengal [3],[4] just on the opposite side of the study area. The foods contributing thiocyanate were cabbage, cauliflower, radish, mustard seeds and leaves, sweet potatoes, turnip, etc., Thiocyanate (SCN) competitively inhibits the iodine-concentrating mechanism of the thyroid. SCN has also been reported to increase iodine efflux, [24] and to interfere with the activity of thyroid peroxidase (TPO) [25] and the incorporation of iodine into thyroglobulin by competing with iodide. [26] It also causes the formation of insoluble iodinated thyroglobulin in thyroid. [27] The present investigation revealed that the mean urinary thiocyanate of the children was 0.857 ± 0.486 mg/dl, which is much higher than the thiocyanate concentration in urine of populations in India without endemic goiter (0.504 ± 0.197 mg/dl). [21]

Thiocyanate not only interferes with iodine utilization in the thyroid gland, but also facilitates the removal/excretion of iodine from the body by inhibiting the iodine uptake into the thyroid. [28] In this study, a significant positive correlation was found between urinary excretion of thiocyanate and iodine [Figure 2]. Thus, SCN present in plant foods consumed by the people of the region may be one of the factors in the pathogenesis of goiter.

In the studied areas, the water used for drinking/cooking was found to be hard, with the local ranges of 210-618 ppm in excess of the threshold value of 200 ppm as defined by Indian drinking water specifications. [29] High mineral content, particularly of magnesium and calcium salts, has been implicated as a goitrogenic factor in several endemic goiter areas. [30],[31] Murray et al. [31] concluded on the basis of their studies that even where the iodine intakes were similar, there was a greater prevalence of visible thyroid glands in people in areas with hard water such as England, than in areas with soft water, such as Scotland. The presence of excess calcium in the colloid of the thyroid follicle causes compactness of thyroglobulin molecules (containing T3 and T4) in follicular cells and their subsequent release in circulation. [32] In a recent study conducted in our laboratory, we found that excess calcium causes enlargement of the thyroid with hypertrophic and hyperplastic changes, retarded TPO and 5'-deiodinase, but enhanced Na-K-ATPase activities, augmented serum total and free T4 and TSH but decreased total and free T3 levels, and a low T3:T4 ratio, resulting in goitrogenesis. [33] A highly significant positive correlation was found between the total hardness of drinking water and goiter prevalence [Figure 3] indicating that hardness of drinking water may have a direct role in the pathogenesis of goiter in the studied region. Thus, in addition to excess iodine and thiocyanate in food, drinking water hardness may have the important role in the pathogenesis of endemic goiter.

Available literature shows that thiocyanate of food or hardness of drinking water or relatively excess iodine intake may decrease thyroid hormone levels, and has less impact on thyroid gland functions; however, the concomitant exposure of those multiple environmental agents may aggravate thyroid disruption as found in the present investigation.

Our results demonstrated that the population of the studied region has a high level of endemic goiter despite the absence of iodine deficiency for the concomitant exposure of at least three progoitrogenic factors viz. thiocyanate, water hardness, and iodine excess. All these act at the different levels of thyroid hormone synthesis causing an interruption in the production of a hormone that in the long run leads to enlargement of the gland by compensatory mechanism developing a goiter. The observations of this study suggests during IDD survey, the effect of concomitant exposure of all the known susceptible progoitrogenic agents present in the environment on thyroid function be evaluated properly.

Acknowledgments

The authors acknowledge the co-operation received from the staff and students of the studied schools.

Financial support and sponsorship

Thanks are due to University of Calcutta for financial assistance (Grant BI 92).

Conflicts of interest

There are no conflicts of interest.

 
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    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2]


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